When creating a custom field in Marketo, how can I create a smart list to see the Null Values

Unfortunately when you create a checkbox field, it default sets everyone in your database to "false" and they become "true" only when the box is checked.

In your case, you'll need to add a "Filled Out Form" filter to your smart list and specify what form the leads would be completing. This should let you know who completed the form and didn't check the box (thereby leaving their default value of "false" the same).

So I can't really get a list of people who filled out the form and actually said "No"? Because if I select the people who filled out the form and No, it also puts everyone who didn't answer as a No as opposed to just giving me the people who checked "No".

I might have misunderstood you, so let me take a step back: what kind of field are you using? A checkbox of a drop down? And if a checkbox, are you only using one, or are there two checkboxes, one for "Yes" and one for "No"?

2 check boxes, one for Yes and one for No. They can choose to select both, neither or 1. If I create a smart list for people who have filled out the form but have checked No = False, it gives me everyone who did not even check the box. I am not able to filter out people who selected No. I can easily filter out the ones that said Yes.

That makes more sense. For any checkbox that's checked, its value becomes "true"

If you want to filter out anyone who checked the "No" box, you'd set "No = True" in your smart list (where No is the name of your field).

Marketo doesn't give me that option. People who filled out the form but did not select the "No" option also get filtered in that List.

I created a form field: Interested in Digital Marketing: Yes No. If people fill out the form and didn't click on either. But when I create a smart list and say Show me People who filled out form. 2nd filter: Interested in Digital Marketing "False" it gives me everyone who didn't check the No box as well as people who checked the No Box. - I only want people who actually selected No. But Marketo isn't giving me an option to filter the ones who selected "No".

If I keep it as true, it gives me people who checked the Yes box only. - This is good.